1. Field of the Invention
The present invention relates to radiation distribution measurement systems for measuring a phase distribution of a beam of radiation and/or a pupil distribution of a projection system.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Advanced optical systems for low k1 lithography require accurate characterization of various imaging parameters to insure that Optical Proximity Correction strategies (OPC) can be maintained. Among these parameters, lens aberrations and illumination profiles are optical column characteristics to be considered. A phase measurement interferometer may be integrated into lithographic projection tools to measure and control tool performance parameters. The primary functionality of such a measurement system is to measure and analyze wave front aberrations across the full image field with high accuracy and speed. In addition to the acquisition of wave front aberrations in terms of Zernike polynomials, detailed measurements of high resolution wave fronts are possible.
In order to measure aberrations of a projection system of a lithographic apparatus, a phase measurement interferometer may be placed on or near the substrate table of the lithographic apparatus. Such an integrated phase measurement interferometer may be based on lateral shearing interferometry using a diffraction grating in front of a camera.
Besides the projection system properties, the detailed shape of the illumination pupil distribution and transmission of the projection system (apodisation) are considered for system operation. The pupil distribution and apodisation can be measured by a sensor that very closely resembles the phase measurement interferometer mentioned above. However, in this case a diffraction grating is not present and is replaced by a small aperture (pinhole) either on the sensor or on the reticle. Of course the sensor is in that case not an interferometer. Below, the term ‘radiation distribution measurement system’ is used as comprising both the phase measurement interferometer and the pupil distribution sensor. The diffraction grating and the pinhole can be integrated in the same sensor by placing the diffraction grating on one portion of the sensor and the pinhole on another portion of the sensor.
Current phase measurement interferometers are configured to measure a light intensity distribution. This light intensity distribution is generated by laser light which is diffracted by a grating structure and subsequently converted to the visible part of the spectrum by means of a conversion layer in front of the sensor. The conversion layer is not always necessary like when, for example, a DUV (deep ultra violet) sensitive camera is used. The grating is replaced by a pinhole so as to measure a pupil distribution. From these intensity measurements, information is retrieved on the aberrations of the projection system (by the interferometer) and on the shape and intensity of the illumination pupil (by the pupil distribution sensor). The light intensity distribution can be measured using a CMOS camera or a CCD camera, or any other camera comprising a plurality of camera pixels.
The light distribution on the camera typically has a maximum intensity near the center of the image of the pupil and a decreasing intensity towards the edge of the image of the pupil. The reason for this is that at the edge of the pupil the light is incident on the camera under a large angle, spreading the light over more pixels than at the center of the pupil.
This non-uniform intensity distribution in combination with the limited dynamic range of typical camera's gives an unwanted degradation of the signal to noise ratio (S/N) towards the edge of the pupil of the projection system.